Inkjet printers have printheads that operate a plurality of inkjets to eject liquid ink onto an image receiving member. The ink may be stored in reservoirs located within cartridges installed in the printer. Such ink may be aqueous, oil, solvent-based, UV curable ink, or an ink emulsion. Other inkjet printers receive ink in a solid form and then melt the solid ink to generate liquid ink for ejection onto the imaging member. In these solid ink printers, the solid ink may be in the form of pellets, ink sticks, granules or other shapes. The solid ink pellets or ink sticks are typically placed in an ink loader and delivered through a feed chute or channel to a melting device that melts the ink. The melted ink is then collected in a reservoir and supplied to one or more printheads through a conduit or the like. In other inkjet printers, ink may be supplied in a gel form. The gel is also heated to a predetermined temperature to alter the viscosity of the ink so the ink is suitable for ejection by a printhead.
A typical full width scan inkjet printer uses one or more printheads. Each printhead typically contains an array of individual nozzles for ejecting drops of ink across an open gap to an image receiving member to form an image. The image receiving member may be a continuous web of recording media, a series of media sheets, or the image receiving member may be a rotating surface, such as a print drum or endless belt. Images printed on a rotating surface are later transferred to recording media by mechanical force in a transfix nip formed by the rotating surface and a transfix roller. In an inkjet printhead, individual piezoelectric, thermal, or acoustic actuators generate mechanical forces that expel ink through an orifice from an ink filled conduit in response to an electrical voltage signal, sometimes called a firing signal. The amplitude, frequency, or duration of the signals affects the amount of ink ejected in each drop. The firing signal is generated by a printhead controller with reference to electronic image data. An inkjet printer forms an ink image on an image receiving surface with reference to the electronic image data by printing a pattern of individual ink drops at particular locations on the image receiving surface. The locations where the ink drops landed are sometimes called “ink drop locations,” “ink drop positions,” or “pixels.” Thus, a printing operation can be viewed as the placement of ink drops on an image receiving surface with reference to electronic image data.
In order for the printed ink images to correspond closely to the image data, both in terms of fidelity to the image objects and the colors represented by the image data, the printheads must be registered with reference to the imaging surface and with the other printheads in the printer. Registration of printheads is a process in which the printheads are operated to eject ink in a known pattern and then the printed image of the ejected ink is analyzed to determine the orientation of the printhead with reference to the imaging surface and with reference to the other printheads in the printer. Operating the printheads in a printer to eject ink in correspondence with image data presumes that the printheads are level with a width across the image receiving member and that all of the inkjet ejectors in the printhead are operational. The presumptions regarding the orientations of the printheads, however, cannot be assumed, but must be verified. Additionally, if the conditions for proper operation of the printheads cannot be verified, the analysis of the printed image should generate data that can be used either to adjust the printheads so they better conform to the presumed conditions for printing or to compensate for the deviations of the printheads from the presumed conditions.
Analysis of printed images is performed with reference to two directions. “Process direction” refers to the direction in which the image receiving member is moving as the imaging surface passes the printhead to receive the ejected ink and “cross-process direction” refers to the direction across the width of the image receiving member. In order to analyze a printed image, a test pattern needs to be generated so determinations can be made as to whether the inkjets operated to eject ink did, in fact, eject ink and whether the ejected ink landed where the ink would have landed if the printhead was oriented correctly with reference to the image receiving member and the other printheads in the printer.
Systems and methods exist for detecting ink drops ejected by different printheads, inferring the positions and orientations of the printheads, and identifying correctional data useful for moving one or more of the printheads to achieve alignment acceptable for good registration in the printing system. The ink drops are ejected in a known pattern, sometimes called a test pattern, to enable one or more processors in the printing system to analyze image data of the test pattern on the ink receiving surface for detection of the ink drops and determination of the printhead positions and orientation. In some inkjet printing systems, printheads are configured to eject a transparent ink onto the ink receiving surface. This transparent ink is useful for adjusting gloss levels of the final printed product and to provide a protective layer over printed areas, if desired. One issue that arises from the use of transparent ink, however, is the difficulty in detecting drops of transparent ink ejected onto an ink receiving surface with an imaging system. Because the transparent inks do not present contrasts with the image receiving surface or other colors ejected by the printer, the known systems and methods for aligning printheads do not enable the transparent ink drops to be detected and the positions and orientations of the printheads ejecting transparent ink to be inferred. Therefore, development of a system and method for aligning printheads that eject transparent ink is a desirable goal.